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Usage of * in sql

* in SQL is a wildcard character with the following usage: Query all columns: SELECT * FROM table_name; Alias all columns: SELECT * AS all_columns FROM table_name; Find specific values in the WHERE clause: SELECT FROM table_name WHERE LIKE '%value %';Use with aggregate functions: SELECT SUM(*) FROM table_name;

Apr 28, 2024 am 11:00 AM

聚合函数

What does like mean in sql

The SQL LIKE operator is used for pattern matching, and the syntax is SELECT * FROM table_name WHERE column_name LIKE pattern; The pattern can use wildcard characters % (any character) and _ (single character), and supports escape characters, character ranges, and NOT LIKE negative matching. But be aware that the LIKE operator is case-sensitive and may be slower for large data tables.

Apr 28, 2024 am 10:57 AM

What does group by mean in sql

The GROUP BY clause is used to group data and perform aggregation operations based on groups. It groups groups of rows with the same value together and calculates summary values or filtered data within each group.

Apr 28, 2024 am 10:54 AM

聚合函数

Usage of loop in sql

LOOP is a control structure in T-SQL that is used to repeatedly execute a block of statements until specific conditions are met. It provides loop control, flexibility, and simplicity, but also requires attention to exit conditions and potential infinite loops.

Apr 28, 2024 am 10:51 AM

How to write not equal to 0 in sql

In SQL, to determine whether a value is not equal to 0, you can use the inequality operator !=, and the syntax is: expression != 0. For example, to find all records that are not equal to 0, you can use SELECT * FROM table_name WHERE column_name != 0;

Apr 28, 2024 am 10:48 AM

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How to express not equal to null in sql

The representation of not equal to NULL in SQL is: IS NOT NULL. This operator checks whether an expression is not equal to NULL and returns a Boolean value: TRUE if the expression is not equal to NULL; otherwise, FALSE.

Apr 28, 2024 am 10:45 AM

How to write not equal to a certain condition in sql

The <> operator is used in SQL to indicate that it is not equal to a certain condition. The example is as follows: SELECT * FROM table_name WHERE column_name <> 'value';

Apr 28, 2024 am 10:42 AM

Usage of all in sql

ALL operator is used to combine all rows of multiple query results, as opposed to DISTINCT which only returns unique rows. It is used to return all rows, ignore duplicates, and handle NULL values. The difference with DISTINCT is that ALL returns all rows, including duplicate rows, while DISTINCT returns only unique rows.

Apr 28, 2024 am 10:36 AM

Usage of multiple fields in sql

There are two ways to use the IN operator on multiple fields in SQL: through a subquery or directly listing multiple values. Subqueries are used to retrieve values from other queries, while multiple values can be listed directly separated by commas. The IN operator checks whether a given value is in the list of specified values.

Apr 28, 2024 am 10:33 AM

The meaning of in in sql

The IN operator in SQL is used to check whether a value belongs to a list of specified values. It can: Check multiple values Optimize performance Improve readability

Apr 28, 2024 am 10:30 AM

What is the command to delete a table in sql

The command in SQL to drop a table is DROP TABLE. This command deletes a table by specifying the table name. The syntax is as follows: DROP TABLE table_name. To delete a table, please enter the command and execute the command in sequence to ensure that the table is not referenced by other objects. It is recommended to back up the table before deleting.

Apr 28, 2024 am 10:21 AM

Usage of using in sql

The USING clause in SQL is used to join tables. It specifies the columns used to join records. It can be used for inner joins, outer joins, cross joins, etc. It can improve query simplicity, readability and performance.

Apr 28, 2024 am 10:18 AM

The meaning of as in sql

The AS keyword in SQL is used to assign the name of an alias or subquery, thereby providing the following benefits: improving query readability and assigning a more descriptive alias. Simplify complex queries and assign aliases to subqueries. Prevent name conflicts and avoid conflicts caused by duplicate table or column names.

Apr 28, 2024 am 10:15 AM

Command to modify table name in sql

Use the ALTER TABLE statement in SQL to modify the table name. The steps are as follows: 1) Confirm permissions; 2) Write the statement (ALTER TABLE RENAME TO ); 3) Execute the statement. Note: The original table must exist, and the new table cannot already exist. Modification may affect dependencies. When the table name contains special characters, a reference identifier must be added.

Apr 28, 2024 am 10:12 AM